In the United States a standard has been proposed for digitally encoded high definition television signals. This standard is essentially the same as the MPEG-2 standard, proposed by the Moving Picture Experts Group (MPEG) of the International Organization for Standardization (ISO). This standard is described in a draft internal standard (DIS) publication entitled "Information Technology--Generic Coding of Moving Pictures and Associated Audio, Recommendation H.262" ISO/IEC 13818-2 DIS, March 1994 which is available from the ISO and which is hereby incorporated by reference for its teaching on the MPEG-2 digital video coding standard.
The MPEG-2 standard is actually many different standards. In MPEG-2 several profiles are defined, each corresponding to a different level of complexity of the encoded image. For each profile, different levels are defined, each level corresponding to a different image resolution. One of the MPEG-2 standards, known as Main Profile, Main Level (MP@ML) is intended for coding video signals conforming to existing television standards (i.e., NTSC and PAL). This standard may be used to encode video images having 480 active lines each with 720 active pixels with a 2:1 interlace scan. When the horizontal and vertical blanking intervals are added to these signals, the result has 525 lines by 858 pixels. When they are decoded, and displayed with a 13.5 MHz display clock signal, these signals produce images that correspond to NTSC-type broadcast images.
Another standard, known as Main Profile, High Level (MP@HL) is intended for coding high-definition television images. Images encoded according to the Main Profile, High Level standard may have as many as 1,152 active lines per image frame and 1,920 active pixels per line. The number of pixels per line and the number of lines per frame are variable. In addition, the Main Profile, High Level signal may be scanned either progressively or using an interlaced format.
The Main Profile, Main Level standard, on the other hand, defines a maximum picture size of 720 pixels per line and 567 lines per frame. At a frame rate of 30 frames per second, signals encoded according to this standard have a data rate of 10,368,000 pixels per second. By contrast, images encoded according to the Main Profile, High Level standard have a maximum data rate of 1,152 * 1,920 * 30 or 62,668,800 pixels per second (the maximum data rate numbers may be found in the MPEG-2 specification at table 8-11). The data rate of the high definition television signals is more than five times the data rate of image data encoded according to the Main Profile Main Level standard. The standard proposed for HDTV encoding in the United States is a subset of this standard, having as many as 1,080 lines per frame, 1,920 pixels per line and a maximum frame rate, for this frame size, of 30 frames per second. The maximum data rate for this proposed standard is still far greater than the maximum data rate for the Main Profile, Main Level standard.
The MPEG-2 syntax defines several layers of data records which are used to convey both audio and video data. For the sake of simplicity, the decoding of the audio data is not described herein. Encoded data which describes a particular video sequence is represented in several nested layers, the Sequence layer, the Group of Pictures layer, the Picture layer, the Slice layer and the Macroblock layer. To aid in transmitting this information, a digital data stream representing multiple video sequences is divided into several smaller units and each of these units is encapsulated into a respective packetized elementary stream (PES) packet. For transmission, each PES packet is divided, in turn, among a plurality of fixed-length transport packets. Each transport packet contains data relating to only one PES packet. The transport packet also includes a header which holds control information to be used in decoding the transport packet.
When an MPEG-2 encoded image is received, a transport decoder (not shown) decodes the transport packets to reassemble the PES packets. The PES packets, in turn, are decoded to reassemble the MPEG-2 bit-stream which represents the image in the layered records, as described above. A given transport data stream may simultaneously convey multiple image sequences, for example as interleaved transport packets. This flexibility also allows the transmitter to switch among formats providing material in 4 by 3 aspect ratio according to one standard and widescreen (16 by 9) material according to another standard.
Because of the wide range of data rates for these different standards, a decoder which can handle all of the video standards defined for MPEG-2 needs a large, high-bandwidth memory or several distributed smaller memories which, when considered together, are at least as large as the single memory. These memories unduly increase the cost of a commercial television receiver. This memory is used to hold the input bit-stream as it is received, to hold two anchor frames of data which are used to decode predictively (P-frame) and bidirectionally predictively (B-frame) encoded data, to hold the frame which is being decoded and to hold data which is being converted from the block-format used for encoding and decoding the image data to a raster-scan format that is used to display the data.